1. Field of the Invention
The present invention relates to a coated steel plate suitable for bonding with a polyamide adhesive prepared by coating on a steel plate, such as a chromated steel plate, a galvanized steel plate, a tin plate, an untreated cold rolled steel plate, a nickel plated steel plate, a vapor aluminum deposited coated steel plate, a chromium thin layer plated steel plate, etc., with a composition comprising a thermosetting phenol resin as described hereinafter and a bisphenol A type epoxy resin.
2. Description of the Prior Art
Hitherto, as materials for metallic containers such as cans for foods and also various kinds of cans, a so-called tin plate, that is a tinned steel plate has been mainly used. The feature of the material is that it can be easily soldered but, on the other hand, the material has disadvantages in that the material lacks printability and the ability to protect the contents in the cans as well as toxicity hazards can occur with the material.
Recently, however, a chromated steel plate and aluminum have been used as materials for tin plate and further the use of an untreated steel plate which has not been subjected to plating or other chemical treatments has recently been investigated.
Processes for producing such a chromated steel plate are a process in which a steel plate is electrolyzed in a bath comprising an aqueous solution of CrO.sub.3 having a comparatively low concentration of less than 100 g/liter having added thereto phenoldisulfonic acid, catecholdisulfonic acid, or a salt of these acids or a small amount of sulfuric acid and a fluoride at an appropriate current density and a process in which a steel plate is electrolytically chromated in an aqueous solution of CrO.sub.3 having a high concentration of higher than 100 g/liter and, after washing the plate with water, the plate is further electrolyzed in an aqueous CrO.sub.3 solution having a low concentration. "Hi-Top" and "Supercoat", trade names made by Toyo Kohan Co., Ltd., are commercially available examples of chromated steel plates produced by the former process and "Cansuper", trade name made by Nippon Steel Corporation, and "Weirchrome", trade name made by National Steel Co., are commercially available examples of chromated steel plates produced by the latter process. Each of these chromated steel plates has a quite thin metallic chromium layer and hydrated chromium oxide layer and the thickness of the layer is usually about 0.05 to 2 mg (as Cr)/dm.sup.2. Another commercially available example of a chromated steel plate is "Hi Nack", trade name made by Nippon Kokan K.K., which is produced by immersing a steel plate in an aqueous solution containing CrO.sub.3 and a reducing agent and, after removal, drying the steel plate by heating to form a coating consisting of hydrated chromium oxide only.
A galvanized steel plate is usually produced by immersing in a molten zinc bath a steel plate which has been cleaned by pickling with sulfuric acid or hydrochloric acid and a tin plate is produced by immersing a pickled steel plate in a molten tin bath or is recently produced by electroplating a steel plate in a bath mainly containing, for example, 76 g of stannous chloride, 25 g of sodium fluoride, 50 g of potassium fluoride, and 45 g of sodium chloride per liter of the bath.
A chromated steel plate has the advantage that it has superior printability as mentioned before but since it has the disadvantage that soldering is not applicable, welding or an adhesive is employed in bonding the plates.
Also, although aluminum is used in some fields due to the rustless property, beautiful appearance, and light weight, the use of aluminum is limited to specific fields due to the high cost of aluminum.
On the other hand, in regard to tin plate, as a result of apprehesion on exhaustion of tin resources, the thickness of the plating tends to be reduced greatly, which tends to make the application of soldering difficult.
An untreated cold rolled steel plate may be lowest in cost but since an untreated cold rolled steel plate rusts readily, sufficient rust proofing is not obtained with an ordinary under coat, and further since sufficient bonding strength is not obtained, an untreated cold rolled steel plate has not been used widely at present.
Many publications on bonding of steel plates with adhesives exist but the results shown are all insufficient in bonding rate, bonding strength, evenness of bonding strength, water proofing property, and heat resistance.
For example, the bonding rate for soldering steel plate in manufacturing metallic cans is about 400 cans/min but in using adhesives the bonding rate must be about 600 to 800 cans/min, the heating and pressing period of time must be about 2 to 3 seconds, and the bonding must be finished in a maximum of about 10 seconds.
Also, polyamide provides excellent adhesives for structural purposes together with a thermosetting resin such as an epoxy resin, a phenol resin, etc., but use of such adhesives is not desirable since three-dimensional hardening is required at bonding and hence a long period of time is required to finish the bonding. For example, the period of time required for hardening such adhesives at bonding is from a few minutes to several tens of minutes at temperatures of about 150.degree. to 250.degree. C, and thus such a bonding rate is unsatisfactory.
On the other hand, in the case of producing cans for food, the cans must retain sufficient bonding strength at temperatures encountered in sterilizing the cans with steam and further the cans are sometimes stored at temperatures from about -20.degree. to about -30.degree. C. In other words, the cans must retain strong bonding strength not only at room temperature (about 20.degree.-30.degree. C) but also over a considerably wide temperature range.